1. Field of the Invention
The present invention relates to a nitride-based semiconductor device and a method of fabricating the same, and more particularly, it relates to a nitride-based semiconductor device having an electrode and a method of fabricating the same.
2. Description of the Background Art
A nitride-based semiconductor laser device has recently been expected as a light source for an advanced large capacity optical disk, and actively developed.
In general, an insulating sapphire substrate is employed for forming a nitride-based semiconductor laser device. When a nitride-based semiconductor layer is formed on the sapphire substrate, however, a large number of defects (dislocations) are disadvantageously formed in the nitride-based semiconductor layer due to large difference between the lattice constants of the sapphire substrate and the nitride-based semiconductor layer. Consequently, the characteristics of the nitride-based semiconductor laser device are disadvantageously reduced.
In this regard, a nitride-based semiconductor laser device employing a nitride-based semiconductor substrate such as a GaN substrate having small difference in lattice constant with respect to a nitride-based semiconductor layer is proposed in general.
FIG. 7 is a sectional view showing a conventional nitride-based semiconductor laser device employing an n-type GaN substrate 101. Referring to FIG. 7, nitride-based semiconductor layers (102 to 110) are grown on a Ga face ((HKLM) plane: M denotes a positive integer) to be improved in crystallinity in a process of fabricating the conventional nitride-based semiconductor laser device. A nitrogen face ((HKL-M) plane: M denotes a positive integer) of the n-type GaN substrate 101 having a wurtzite structure is employed as the back surface, so that an n-side //electrode 112 is formed on this back surface of the n-type GaN substrate 101. The fabrication process for the conventional nitride-based semiconductor laser device is now described in detail.
As shown in FIG. 7, an n-type layer 102 consisting of n-type GaN having a thickness of about 3 μm, an n-type buffer layer 103 consisting of n-type In0.05Ga0.95N having a thickness of about 100 nm, an n-type cladding layer 104 consisting of n-type Al0.05Ga0.95N having a thickness of about 400 nm, an n-type light guide layer 105 consisting of n-type GaN having a thickness of about 70 nm, an MQW (multiple quantum well) active layer 106 having an MQW structure, a p-type layer 107 consisting of p-type Al0.2Ga0.8N having a thickness of about 200 nm, a p-type light guide layer 108 consisting of p-type GaN having a thickness of about 70 nm, a p-type cladding layer 109 consisting of p-type Al0.05Ga0.95N having a thickness of about 400 nm and a p-type contact layer 110 consisting of p-type GaN having a thickness of about 100 nm are successively formed on the upper surface (Ga face) of the n-type GaN substrate 101 having a thickness of about 300 μm to about 500 μm.
Then, a p-side electrode 111 is formed on a prescribed region of the upper surface of the p-type contact layer 110. The back surface of the n-type GaN substrate 101 is polished until the thickness of the n-type GaN substrate 101 reaches a prescribed level of about 100 μm, and an n-side electrode 112 is thereafter formed on the back surface (nitrogen face) of the n-type GaN substrate 101. Finally, the n-type GaN substrate 101 and the layers 102 to 110 are cleft thereby performing element isolation and forming a cavity facet. Thus, the conventional nitride-based semiconductor laser device shown in FIG. 7 is completed.
In the conventional nitride-based semiconductor laser device shown in FIG. 7, however, the n-type GaN substrate 101 is so hard that it is difficult to excellently perform device isolation and form the cavity facet by cleavage. In order to cope with such inconvenience, a method of mechanically polishing the back surface of the n-type GaN substrate 101 before the cleavage step for reducing irregularity on the back surface thereby excellently performing element isolation and forming the cavity facet is proposed. This method is disclosed in Japanese Patent Laying-Open No. 2002-26438, for example.
In the aforementioned conventional method disclosed in Japanese Patent Laying-Open No. 2002-26438, however, stress is applied in the vicinity of the back surface of the n-type GaN substrate 101 when the back surface of the n-type GaN substrate 101 is mechanically polished. Therefore, microscopic defects such as cracks are disadvantageously formed in the vicinity of the back surface of the n-type GaN substrate 101. Consequently, contact resistance between the n-type GaN substrate 101 and the n-side electrode 112 formed on the back surface (nitrogen face) thereof is disadvantageously increased.
Further, the nitrogen face of the n-type GaN substrate 101 is so easily oxidized that the contact resistance between the n-type GaN substrate 101 and the n-side electrode 112 formed on the back surface (nitrogen face) thereof is disadvantageously increased also by this.